Current transformer with internal error compensation



Oct. 13, 1970 E P. N; MILJANIC 3,534,247

CURRENT TRANSFORMER WITH INTERNAL ERROR COMPENSATION Filed May 15, 1968 2 Sheets-Sheet 1 SUEDE J I .38 m Su ly I /3 v I i 1 f Z7 Pan/E2 (7 1200 5 uppl AMP! 0'1:

P /9R N. mun/v10 B7 HTT NEy5 Oct. 13, 1970 P. N. MIL JANIC 7 CURRENT TRANSFORMER WITH INTERNAL ERROR COMPENSATION Filed May 15, 1968 2 Sheets-Sheet 2 ('utPEL/f A MP: 1/752 I v I auzasu I l I I r/wE/vroR PET/9R M mama 1c 9 77 NE ys United States Patent ABSTRACT OF THE DISCLOSURE Due to magnetizing losses in the working core of a current transformer, the relationship between currents in the primary and secondary windings does not correspond accurately to the turns ratio. To correct for such inaccuracy, an additional core is provided for sensing the magnetic flux corresponding to the magnetizing losses. A further winding linking this core is connected in parallel with the secondary winding of the current transformer, and a feedback amplifier supplies a current through this further winding, which current creates zero residual flux in the sensing core. The power required for such correction circuit is derived from the actual currents or portions of currents forming the ratio, either through an auxiliary power supply transformer connected in series with the primary or the secondary windingof the current transformer or through the further winding, which in this case is splitinto two sections and acts together with the additional core as both an ampere turn sensing device and a current auto transformer.

The resulting corrected current transformer has only two input and two output terminals and thus forms a passive device requiring no auxiliary external source of power.

The present invention relates to a current transformer with internal error compensation.

The error of a current transformer, which is due primarily to the magnetizing losses in the working core, is proportionately very large when the primary and secondary windings impose only a few ampere turns on the core, as is particularly the case in the bushing-type. In order to keep the magnetizing losses low, the use of very large, high permeability cores has been proposed. Biasing or higher frequency excitation techniques have also been proposed, but all these expedients tend to be either cumbersome or expensive or both.

Features of the invention will be apparent from the following specific description which is provided by way of example only. In the accompanying drawings:

FIG. 1 shows a first embodiment of a current transformer circuit according to the invention;

FIG. 2 is a fragment of FIG. 1 showing a modification;

FIG. 3 is a diagrammatic view showing an example of a transformer used in the circuit of FIG. 1;

FIG. 4 is another example of a transformer for use in the circuit according to FIG. 1;

FIG. 5 is a second embodiment of a current transformer circuit; and v 1 7 FIG. 6 showsa third embodiment of a current transformer circuit according to the invention.

Referring to FIG. 1, the current transformer circuit 10 comprises an input circuit 11 connected to input terminals 12and 13 andanoutput circuit 14 connected to output terminals 15 and16.jA burden 17, which may be an ammeter and which does not form part of the invention, is shown connected to the output terminals 15 and 16.

The current transformer 10 includes two magnetic cores, namely a working core and a sensing core 21.

3,534,247. Patented Oct. 13, 1970 "ice A primary winding 22 connected to the input terminals 12 and 13 creates a magnetic flux in both the working core 20 and the sensing core 21, and similarly a secondary winding 23 connected across the output terminals 15 and 16 links both cores 20 and 21. A further winding 24, which has the same number of turns as the secondary winding 23, links only the sensing core 21. Two examples for the physical arrangement of the two cores 20 and 21 and the three windings 22 to 24 are described below in connection with FIGS. 3 and 4.

One end 25 of the further winding 24 is connected to the output terminal 15, while the other end of the winding 24 is connected to a first output terminal 26 of a current amplifier 27, the second output terminal 28 of which is connected to the output terminal 16 of the output circuit 14. Thus, the series connection of the winding 24 and the current amplifier 27 is connected in parallel with the secondary winding 23 and with the burden 17. A control input terminal 29 of the current amplifier 27 is connected to the end 25 of the winding 24, while the other control input terminal 30 of the amplifier 27 is connected to the other end of the winding 24 and thereby to the output terminal 26 of the amplifier 27 itself. Ideally, the current amplifier 27 has an infinite input impedance and infinite gain. In more precise terms, the transconductance between the input voltage and the output current of the amplifier 27 ideally approaches infinity.

The power required for the amplifier 27 is taken from a power supply 35 the input of which is formed by the secondary winding 36 of a power supply transformer 37. In the embodiment of FIG. 1, the primary winding 38 of the power supply transformer 37 is connected in series with the primary winding 22 of the input circuit 11.

To describe the principle of operation of the current transformer circuit shown in FIG. 1, it is assumed that the input circuit 11 carries a primary current I1, the secondary winding 23 carries a secondary current I2, the turns ratio between the primary and secondary windings 22 and 23 is 1:11 and the output current I0 is the current flowing through the burden 17. The relationship between the primary current I1 and the secondary current 12 is given by the following equation:

I 1 I 2- Lu (1) wherein l is the magnetizing current covering the losses in the working core 20. No such losses occur in the sensing core 21, since the operation of the circuit is such as to create zero residual flux in this core, as will be understood from the following description.

The residual zero magnetic flux in the sensing core 21 can be conceived as composed of .a positive magnetic flux +F,u and a magnetic flux F,u. of equal value and opposite phase. The positive magnetic fiux +F corresponds to the difference between the ampere-turns created by the primary current I1 through the primary winding 22 linking the sensing core 21 and the ampere-turns created by the secondary current I2 through the secondary winding 23 linking the sensing core 21, and therefore equals the magnetizing losses mentioned above. This positive magnetic flux +F/L induces a voltage across .the winding 24 which voltage appears at the input terminals 29 and 30 of the amplifier 27 which in turn feeds a current through the winding 24. Due to this feedback connection of the amplifier 27 and due to the fact that the amplifier has a very high gain, ideally infinite gain, the current through the winding 24 will create the above-mentioned negative flux F,u. so that the residual flux in the sensing core 21 approaches zero and thus the voltage induced in the winding 24 similarly approaches zero. Since the number of turns of the winding 24 equals that of the secondary winding 23,

the current creating the negative magnetic flux F,u. is equal to the magnetizing current I;/.. If the number of turns of the winding 24 were smaller than that of the secondary winding 23, the same result could be achieved by a resistance network appropriately dividing the current flowing through the winding 24.

The resulting current I led to the burden 17 equals the sum of the secondary current 12 and the magnetizing current In, giving Io=l2+l,u. (2)

and with respect to Equation 1 ,so that the output current I0 is proportional to the primary current I divided by the turns ratio 11 between the secondary and primary windings 23, 22, as desired.

The power required for the current amplifier 27 is equal to the burden voltage across the output terminals 15 and 16 multiplied by the magnetizing current I and is usually below one watt. In the embodiment of FIG. 1, the power supply 35 is coupled to the input circuit 11 and forms part of the current transformer circuit 10.

In this circuit the winding 24 serves two functions: firstly it carries the output current of the amplifier 27, which creates the negative magnetic flux F,u in the sensing core 21, and secondly it senses the residual flux in the sensing core 21 to supply the input voltage for the amplifier 27. Due to the finite gain of the amplifier 27, and also due to the voltage drop across the winding 24, the current I added to the secondary current I2 is slightly smaller than the magnetizing current.

In the modified circuit according to FIG. 2, an auxiliary winding 40 with any convenient number of turns is provided to link only the sensing core 21 and is connected to the amplifier input terminals 29 and 30. In this embodiment, the detection of the residual flux in the sensing core 21 is performed by the auxiliary winding 40 and the negative magnetic flux F/.L is created by the winding 24. This functional separation reduces current loss in the winding 24 and also avoids voltage loss across the input terminals 29 and 30 of the amplifier 27 due to the voltage drop at the current carrying winding 24.

FIG. 3 shows a first example for the physical embodiment of the magnetic cores and the arrangement of the various windings thereon. According to this example, both the working core 20 and the sensing core 21 take the shape of annular laminated magnet cores 20a and 21a disposed adjacent each other and separated from each other by an air gap sufficient to avoid magnetic cross-coupilng. Usually, the cross-section of the sensing core 21a will be much smaller than that of the working core 20a. The primary and secondary windings 22 and 23 are wound around the adjacent limbs 42 and 43 thereby linking both cores 20a and 21a, whereas the further winding 24 andif present-the auxiliary winding 40 are wound only around the sensing core 21a.

in the second example shown in FIG. 4, the working core and the sensing core take the shape of toroidal laminated magnetic cores 20b and 21b, with the sensing core 21b nested inside the working core 20b. FIG. 4 shows only a short part of the complete torus. The primary and secondary windings 22 and 23 are wound on the exterior of the working core 20b, thereby linking the working core 20b as well as the sensing core 21b. The further winding 24 and-if present-the auxiliary Winding 40 are wound around the exterior of the sensing core 21b but are disposed inside the working core 2012, thereby only linking the sensing core 21b. A transformer according to the example of FIG. 4 is described in more detail in US. Pat. No. 3,153,758 to N. L. Kusters et al. issued Oct. 20, 1964.

The second embodiment of a current transformer circuit according to the invention shown in FIG. differs from the one shown in FIG. 1 in that the primary winding 38 of the power supply transformer 37 is disposed in the output circuit 14 and is connected in series with the secondary winding 23 of the current transformer. In this connection, the power supply transformer 37 imposes an additional load on the current transformer and therefore increases the current In due to increased magnetizing losses, which current has to be furnished by the further winding 24. The circuit of FIG. 5, however, has an advantage from the design point of view. The size of the power supply transformer 37 can be reduced, because its primary winding 38' carries the secondary current 12 rather than the much heavier primary current I1.

Instead of using one further winding 24 for both creating the negative magnetic fiux in the sensing core 21 and sensing the residual flux in this core, the modification of FIG. 2 could be applied in the circuit of FIG. 5 using an auxiliary sensing winding '40. As is obvious, the physical embodiment of the working core 20 and the sensing core 21 and the arrangement of the windings on these cores can be made according to the example shown in FIG. 3 or to that shown in FIG. 4.

In the circuits of FIGS. 1 and 5, the amplifier output power is positive, i.e. the amplifier is supplying power that is added to the power supplied through the secondary winding 23. In a third embodiment shown in FIG. 6, the amplifier output power is negative, as will be understood from the following description.

According to FIG. 6, the turns ratio between the secondary and the primary windings 23' and 22 is n which is slightly smaller than the nominal transformer ratio 12. Hence the secondary current 12' is larger than the corresponding secondary current I2 in the circuits of FIG. 5

The further winding is split into two sections 24a and 24b connected in series with each other, and the secondary winding 23' is connected to the junction point 44 of the two sections 24a and 24b. The number of turns of the section 24b equals that of the secondarywinding 23'. The total number of turns of both sections 24a and 24b corresponds to the nominal transformer ratio, leaving for the section 24a a number of turns which is related to the number of turns of the primary winding 22 by the ratio (nn'). The power supply 35 is directly connected to the output terminal 26 of the amplifier 27, thereby forming together with the amplifier 27 an active impedance which absorbs the portion of the secondary current 12 that exceeds the desired output current I0. -In other words, the power is supplied by means of the current I2Io flowing through the winding section 24b to the power supply 35. This assumes that the current I2 is without error and that there are no voltage drops due to internal impedances. 'If the current 12' is in error, the amplifier 27 will force an additional current into the section 24b to correct for such error.

The control mechanism of the winding 24a, 24b which acts as an auto-transformer in combintaion with the amplifier 27 and the power supply 35 is basicall the same as in the circuit according to FIG. 5. As can easily be understood, the positive magnetic flux +F in the sensing core 21 has not changed, since the magnetization of the working core 20 is assumed to be the same which results in the same magnetizing losses. The negative magnetic flux -F,u. created by the further winding sections 24a and 2412 can be expressed by the following equation -F .taI0(N-n')-(I2'I0)n' (5) wherein the term I0(n-n') represents the ampere turns in the section 24a and the term (l2'Io ):n represents the ampere turns in the section 24b. By simplifying Equation 5 and also substituting in it 12 for I2 in accordance with Equation 4, the negative magnetic flux -F,u in the sensing core 21 becomes 'F,uom(IoI2) (6) or in the light of Equation 2 -F,uart.l,u. (7)

Remembering that the turns ratio of the winding 24 in the circuit of FIG. 5 equals n, the expression given in Equation 7 also represents the negative magnetic flux created by the further winding 24 in the embodiment of FIG. 5, which flux has been explained as causing zero residual flux in the sensing core 21.

Again and still referring to FIG. 6, instead of connecting the input terminals 29 and 30 of the amplifier 27 to the ends 25 and 26 of the winding 24a, 2411, an auxiliary winding 40 with any convenient number of turns can be provided as shown in FIG. 2. As before, the physical arrangement of the working and sensing cores and the various windings can be made in accordance with FIG. 3 or 4 with the additional feature of an intermediate tap on the further winding 24.

I claim:

1. A current transformer comprising:

(a) a first core;

(b) a second core magnetically separate from said first core;

(c) a primary winding linking both said cores to create a first magnetic flux in said first core and a second magnetic flux in said second core;

(d) a secondary winding linking both said first and said second cores;

(e) output terminals for connection to an external burden and connected across said secondary wind- (f) a further winding linking said second core, said further winding being connected across said output terminals;

(g) means for generating current through said further Winding for creating in said second core a magnetic flux substantially equal and opposite to said second flux; and

(b) means for supplying power to said current generating means, said power supply means having input means connected in series with one of said windings;

(i) wherein said current generating means includes amplifier means having input means and output means, said output means being connected in series with said further winding and said output means being connected to sense any residual magnetic flux in said second core.

2. A current transformer as in claim 1, wherein said power supply input means includes a transformer having its primary winding connected in series with one of said windings linking both said cores.

3. A current transformer as in claim 2, wherein the primary winding of said power supply transformer is connected in series with the primary winding of said current transformer.

4. A current transformer as in claim 2, wherein the primary winding of said power supply transformer is connected in series with said secondary winding.

5. A current transformer as in claim 2, wherein the number of turns of said further winding is equal to the number of turns of said secondary winding.

6. A current transformer as in claim 1, wherein said further winding includes a first section and a second section connected in series to said first section, said first section being connected in parallel with said secondary winding, and wherein said power supply input means is connected to said first section.

7. A current transformer as in claim 6, wherein the number of turns of said first section is equal to the number of turns of said secondary winding.

8. A current transformer as in claim 1, wherein said amplifier input means is connected across said further winding.

9. A current transformer as in claim 1, further comprising an auxiliary winding linking said second core, and wherein said amplifier input means is connected across said auxiliary winding.

10. A current transformer as in claim 1, wherein said second core is nested within said first core, said primary and secondary windings being wound onto said first core and said further winding being wound onto said second core.

References Cited UNITED STATES PATENTS 1,995,530 3/1935 Askey 32348 X 2,244,386 6/1941 Camilli 323-48 3,153,758 10/1964 Kusters 324 I. D. MILLER, Primary Examiner G. GOLDBERG, Assistant Examiner US. Cl. X.R. 

